Looking for SD Card – Everything You Need to Know

Have you been in a situation looking for your next SD card, comparing different brands and models, and trying to choose the best for your needs? Then you realize that on some of the SD cards you see the speed in MB/s, on others you see it with the “x-rating” (like 633x, 1000x, 2000x). Furthermore, some cards have shown the speed class number (usually nowadays it is C10), and some have a U1 or U3 sign. Some are marked with SDHC, and other SDXC I or SDXC II. And some cards have V30, V60 or V90 on them… So what do all these numbers mean?

In a previous article, I already talked about the different card formats and standards, comparing speeds and technologies behind. In this article I’ll try to bring some more light on the SD cards, as they are still the most used memory cards today. I will explain what the different numbers mean and give you today’s most popular SD cards (2019-2020) compared. If you don’t feel like reading all the details and only want to see the card comparison , you can skip to this link. For the rest of you, happy reading!

Some History

Secure Digital standard was introduced in August 1999 as a joint between SanDisk, Panasonic (Matsushita Electric) and Toshiba. It was intended to be an improvement over MultiMediaCards (MMC) cards and become the new industry standard. The three companies formed SD-3C, LLC –  a company that licenses and enforces intellectual property rights associated with SD memory cards. In 2000, the three companies also created SD Card Association (SDA) which now has over 1000 member companies.

SD Cards

Secure Digital, officially abbreviated as SD, is a proprietary non-volatile memory card format developed by the SD Card Association (SDA) for use in portable devices. It includes five card families available in three different sizes: Standard-Capacity (SDSC), High-Capacity (SDHC), eXtended-Capacity (SDXC), Ultra-Capacity (SDUC) and SDIO(combines input/output functions with data storage).

SD (SDSC)

Year: 1999
Capacity: up to 2Gb
File System: FAT16

Secure Digital High Capacity (SDHC)

Year: 2006
Capacity: more than 2 Gb up to 32 Gb
File System: FAT32

Secure Digital eXtended Capacity (SDXC)

Year: 2009
Capacity: more than 32 Gb up to 2 Tb
File System: FAT32/exFAT

Secure Digital Ultra Capacity (SDUC)

Year: 2018
Capacity: more than 2 Tb up to 128 Tb
File System: exFAT

Secure Digital Input Output (SDIO)

A SDIO card is an extension of the SD specification to cover I/O functions. SDIO cards are only fully functional in host devices designed to support their input-output functions (typically PDAs like the Palm Treo, but occasionally laptops or mobile phones).

SD Card Speed

SD cards are primary measured by their speed, and more specifically, by their sequential read or write speed. The sequential speed is more relevant for storing and retrieving large files, such as images and multimedia. Small data falls under the much lower speed limit of random access.

Some manufacturers today, but mostly in the early days, the speed was specified as a “times” (“×”) rating. This comes from comparing the average speed of reading data to that of the original CD-ROM drive. The “times” factor was superseded by the Speed Class Rating, which guarantees a minimum rate at which data can be written to the card.

Today, card manufacturers improve SD card speed by increasing the bus rate. The bus rate is the frequency of the clock signal that sends information into and out of the card.

Ultra High Speed (UHS)

It is available on some SDHC and SDXC cards. There are several UHS speeds:

UHS-I

Bus speed: up to 50Mb/s (half and full duplex), or 104 MB/s (half-duplex).

UHS-II

Bus speed: up to 156 MB/s (full-duplex) or 312 MB/s (half-duplex)

UHS-III

Bus speed: FD312 provides 312 MB/s and FD624 doubles that. Both are full-duplex. The physical interface and pin-layout are the same as with UHS-II, retaining backward compatibility.

SD Express

The SD Express bus uses a single PCIe lane to provide full-duplex 985 MB/s transfer speed.

Card Classes

Speed Class

The Speed Class rating C2, C4, C6 and C10 represents that the card supports the corresponding number of megabytes per second as a minimum sustained write speed. Class 10 also uses a High Speed bus mode (U1).

By comparison, the older “×” rating measured maximum speed under ideal conditions, and wasn’t clear whether this was read speed or write speed.

UHS Speed Class

UHS-I and UHS-II cards can use UHS Speed Class rating with two possible grades: class 1 for minimum read/write performance of at least 10 MB/s (‘U1’ symbol featuring number 1 inside ‘U’) and class 3 for minimum write performance of 30 MB/s (‘U3’ symbol featuring 3 inside ‘U’)

Video Speed Class

Video Speed Class defines a set of requirements for UHS cards to match the modern MLC NAND flash memory and supports progressive 4K and 8K video with minimum sequential write speeds that vary from 6 to 90 MB/s. The graphical symbols use ‘V’ followed by a number designating write speed (V6, V10, V30, V60, and V90).

Application Performance Class

Application Performance Class is a newly defined standard from the SD Specification 5.1 and 6.0 which not only define sequential Reading Speeds but also mandates a minimum IOPS for reading and writing. Class A1 requires a minimum of 1500 reading and 500 writing operations per second, while class A2 requires 4000 and 2000 IOPS.[86] A2 class cards require host driver support as they use command queuing and write caching to achieve their higher speeds. If used in an unsupported host, they might even be slower than other A1 cards.

“×” Rating

As mentioned before, the “×” rating was used by some card manufacturers and has been made obsolete by speed classes.

Compatibility

Most common, the newer versions of the specification provide backward compatibility and accept older SD cards. For example, SDXC host devices accept all previous families of SD memory cards, and SDHC host devices also accept standard SD cards.

Older host devices generally do not support newer card formats, even if they might support the bus interface used by the card.

Card Comparison

The table below shows how different speed classes correlate to the minimum sequential write speed. We have the 10Mb/s line where all classes are presented. For the regular speed class this is the last one, for the UHS class it is the beginning, and the video class starts from 6Mb/s and goes all the way to 90Mb/s minimum write speed for V90 class.

Currently on the Market (2019-2020)

In the table below I compare the most important SD Cards today. I picked to compare the 128Gb versions (with two exclusions, as I didn’t find 128Gb versions of these). Compared are the Data transfer speeds – maximum read and write speed and also the very important Minimum write speed. I have also prepared for you an extended version of the table, which includes the corresponding speed class and x-rating, as (if) specified by the manufacturer. You can download it as PDF or see it as an image.

Recommendations

Which of the cards I would recommend? Of course it depends on different factors. For example, Sony A7RIII is a 42Mpx camera. It has two card slots, but on eis UHS I and one is UHS II. If you shoot RAW or RAW+JPG simultaneously on both, it pretty much doesn;t matter how fast card you put in slot one, as it will wait to write the data on slot two as well. If you record RAW on slot one and JPG on slot two, then it makes sense to put faster UHS two card in slot one. As you can guess the variations are many, so you have to pick the cards according to your style and preferences of shooting.

For cameras with one or two UHS II slots I recommend the SONY SF-G Tough cards, as they have 300Mb/s Max Read speed, 299Mb/s Write speed and are V90 compliant, which guarantee 90Mb/s Min Write speed. If the price matters, then look at the Lexar Professional 2000x – it is often on discount (currently $137.99 on Amazon).

For cameras with one UHS II and one UHS I slots I think you can save some money and get a lower speed class for the UHS II slot – like Sony SF-M Tough for example (It is only $58 on Amazon at the moment) or Pro Grade Digital (V60 version) – currently $63 on Amazon after applying a coupon.

For cameras with one or two UHS I slots, I would recommend SanDisk Extreme PRO. I also recommend using this for the UHS I slot of cameras with mixed speed card slots.

What are your preferred cards? Did you have issues with failing cards? Please share in the comments below. I will collect your experience and will make a table with the most “unreliable” cards on the market.